Almost all microbial conversions in nature take place by means of microbial mixed biocenoses (a mixed biocenosis is the symbiosis of different microorganisms). Also, many industrial biological processes, e.g., the biological purification of waste water or the production of sourdough are based on the metabolism of microbial mixed biocenoses. It is known that mixed biocenoses develop over a very long period of time, in comparison to the generation time of the individual species, to very mature and stable biocenoses in which the organizational level, which can be expressed, for example, as the ratio of biomass to productivity, is high. This is particularly true for biocenoses whose environmental conditions remain relative constant, as is the case in the intestine of warm blooded animals. Stable biocenoses generally exhibit a plurality of control mechanisms which buffer changes of external conditions. Species spectrum and population of individuals of a biocenosis oscillate in their climax stage about an optimum point. If, for example, the amount of nutrients increases, essentially only the most populous allochthonous microorganisms will multiply temporarily while the highly specialized autochthonous ones remain almost constant. These autochthonous microorganisms have frequently become so specialized that they require growth factors, (sometimes called supplines) in order to live.
Growth factors are substances which are associated with the basic constituents of the cell and which can not be synthesized by individual organisms from the simple building blocks. They are amino acids, purines, pyrimidines, organic acids, carbohydrates and vitamins (See Starr, M. P., Stolp, H, Truper, H. G., Boleros, A and Schliegel, H. G., Eds: The Prokaryotes: A Handbook on Habits, Isolation and Identification of Bacteria, Springer, N.Y., 1981; Davis, B. D., Dulbecco, R., Eisen, H. N., Ginsberg, H. S., Wood, B.: Microbiology, Harper Int. Ed. N.Y., 6th Ed. 1970). Carbohydrates can be both nutrients and growth factors. Carbohydrates which occur particularly infrequently can often be considered as growth factors. Growth factors differ distinctly from nutrients as regards their function and concentration. They correspond to vitamins in animal and human nourishment. Frequently, final or intermediary products of the metabolism of one species in microbial mixed biocenoses serve as growth factors for another species, which results in a control mechanism (c.f. Schlegel, "Allgemeine Microbiologie", 5th edition, 1981, Stuttgart, Verlag Thieme, page 169). Another control mechanism consists in that the individual members of a microbial biocenosis react with different sensitivities to inhibitory substances stemming either from other members of the microbial biocenosis or also from a higher symbiotic partner, e.g. a host plant.
In biocenoses, the individual functions are exercised by certain members of this biocenosis which thus fill a certain ecological niche. As a rule, the same ecological niche can be filled by several species. It has been found that in microbial mixed biocenoses, individual bacterial species with positive qualities can be purposefully stimulated and/or individual members with certain negative qualities can be purposefully suppressed if use is made of the fact that many autochthonous bacterial species of stable microbial mixed biocenoses exhibit a different sensitivity to growth factors and/or to biotic inhibitory substances.
Frequently, growth factors of one species can have an inhibiting action in certain concentrations on other species. The same growth factor can both inhibit and stimulate the same species.